Cellulose-sulfur composition which resists chain shortening



United States Patent 3,464,840 CELLULOSE-SULFUR COMPOSITION WHICH RESISTS CHAIN SHORTENTNG Donald M. MacDonald, Hawesbury, Ontario, Canada, as-

signor to Canadian International Paper Company, Moutreal, Quebec, Canada, a corporation of Canada No Drawing. Filed Jan. 4, 1965, Ser. No. 423,317 Int. Cl. C08b 27/64 US. Cl. 106162 3 Claims ABSTRACT OF THE DISCLOSURE A stable composition comprising a cellulosic material, alkali, and sulfur in an amount of from about 0.1% to 2.0% by weight of the cellulosic material.

This invention relates to the production of cellulose having greater average chain lengths. More particularly, it relates to a method of inhibiting the breaking down of cellulose chains during aging or other reactions.

The processes "required for the manufacture of articles of commerce from regenerated cellulose or from cellulose ethers may be divided into several distinct steps. Usually, although the present invention is not intended to be restricted to this process, the following steps are followed:

(a) Purified cellulose, either wood pulp or cotton linters, in sheet form, is placed in strong (about 18%) sodium hydroxide solution. Subsequently, the sheet may be broken up by vigorous agitation (slurry process) or allowed to remain undisturbed in the caustic solution (sheet steeping process). After a predetermined time, varying from seconds to hours, the excess caustic is drained off and the cellulose mass pressed to remove further caustic. The pressed mass, now known as alkali cellulose, is shredded and broken up into small fragments or almost to individual fibers. This shredding step can also be carried out at the end of step (b) infra;

(-b) The alkali cellulose is allowed to age for a predetermined time and at a predetermined temperature;

(0) The aged alkali cellulose is treated with carbon disulfide (xanthated) and dissolved at the end of the reaction time in dilute caustic soda (sodium hydroxide solution). The solution is known as viscose. Alternatively, the alkali cellulose can be treated with ethylene oxide to produce hydroxyethyl cellulose, with chloroacetic acid to produce carboxymethyl cellulose, or in other manners known to those skilled in the art.

Cellulose, when treated with strong alkali, undergoes a complex reaction with atmospheric oxygen and with alkali. The main feature of this reaction is a shortening of the molecular chain length by breakage at random positions. This reaction continues in the above process until such time as the cellulose derivative is dissolved in water or in more dilute caustic soda, or is freed from strong caustic by any means. The extent of the chain shortening is governed by the time and temperature used in step (b). Even if step (b) is eliminated, chain shortening occurs at all times in steps (a) and (c) when the cellulose is in contact with strong caustic.

In many instances, it is desirable that the chain shortening, or aging reaction, be minimized. Such instances occur, for example, in the production of cellulose sausage casings, in the production of high wet modulus rayons ofthe Toramomen type, and in the production of cellulose ethers when prepared from cellulose following a strong caustic treatment and for use as viscosity increasers for aqueous solutions. In the first two instances, very desirable strength properties are obtainable if the average cellulose chain length is kept as great as possible and, in the last instance, the longer the average chain length the less cellulose ether is required for a given solution viscosity increase.

3,464,840 Patented Sept. 2, 1969 (The term aging reaction as used herein refers not just to step (b) but to all chain shortening processes due to the actions of alkali and atmospheric oxygen in any procedure in which cellulose is treated with solutions of caustic stronger than about 7%. For economic reasons, the preferred base is sodium hydroxide, rather than hydroxides of lithium, potassium, or caesium, but these can also be used.)

It is known that alkali metal sulfites (US. Patent No. 1,857,948, to R. Dosne, dated May 10, 1932), certain heavy metal salts (Entwistle, Cole and Wooding, Text. Res. 1., 19,609-624 (1949)); Sihtola and Bostrom, Paper and Timber, 34, 23-27 (1952), sodium borohydride (Swedish Patent 162,095 to I. lullander dated Feb. 11, 1958), polyhydric phenols (German patent application No. ST 3147 filed Mar. 12, 1951 by Dr. Hermann Staudinger), sodium trithiocarbonate (US. Patent 2,473,954 dated June 21, 1949 to G. M. A. Kayser), and gluconic or mucic acids (Canadian Patent 563,087 dated Sept. 9, 1958 to W. R. Saxton) can inhibit chain shortening in the aging reaction.

None of these materials has enjoyed wide use as all suffer from objections. Sodium sulfite is soluble in steeping lye and must be added after pressing unless very large quantities are used. Sodium trithiocarbonate also suffers from this objection and is not chemically stable. Sodium borohydride is also unstable and, on decomposition, generates hydrogen causing an explosion hazard. Poly-hydric phenols turn brown in caustic and cause undesirable colors in the final product. The heavy metal salts may also give objectionable color and can cause difiiculties in filtration of solutions of the products. Gluconic and mucic acids are primarily intended to combat the accelerating effect of accidentally introduced catalytic heavy metal ions, and these acids are soluble in steeping lye.

It is a principal object of this invention to provide a really effective inhibitor of chain shortening during the aging reaction which is not subject to the noted objections.

It has now been found that sulfur can be added to cotton linters or wood pulp before either is subjected to sheet or floc formation and that the sulfur is retained by the pulp. The sulfur is not soluble in a steeping lye and there is accordingly no contamination problem. Moreover, it does not cause difficulties in filtration of solutions of cellulose xanthate and has no detectable harmful elfect on the color or strengthvof the final product. The sulfur can also be added to caustic prior to bringing the caustic into contact with cellulose or to alkali cellulose prior to bringing the alkali caustic into contact with carbon disulfide in xanthation. But, for best results, it should be added as early as possible, preferably no later than at the start of the shredding operation, so as to assure even dispersal of the sulfur throughout the mass.

Quantities of sulfur ranging from about 0.1 to 2.0% based on the weight of cellulose effectively inhibit aging. The preferred amount is 1% by weight.

The following examples are not intended to restrict the invention in any way, but are offered merely as illustrations of the effectiveness of the present invention. The figures under D.P. (degree of polymerization) indicate the average number of monomer units of cellulose per chain at the specified time. The viscose viscosity is well known to be proportional to the average chain length, so that these figures give an idea of the overall effect of the sulfur.

Example I A sulfite wood pulp of dissolving grade in sheet form (specifically, Tenacell grade pulp manufactured at the Kipawa Mill of Canadian International Paper Company) was treated with a solution of sulfur in carbon disulfide. The added weight of sulfur was 1% based on cellulose.

This pulp was dried until no trace of carbon disulfide remained and was subsequently converted to alkali cellulose by sheet steeping in 18% sodium hydroxide solution. After draining and pressing, the alkali cellulose contained 32% cellulose. A sheet of this alkali cellulose was torn into small pieces and thoroughly dispersed in an about 5% aqueous acetic acid solution (this step causes cellulose to be reformed from the alkali cellulose). The D.P. of the cellulose after thorough washing was determined from its viscosity in cuprammonium hydroxide solution.

The bulk of the alkali cellulose was shredded for 60 minutes and a second sample of cellulose was regenerated for D.P. determination. A third sample was regenerated after aging for 24 hours, after which time the remaining alkali cellulose was treated with carbon disulfide (about 33% CS and converted to cellulose xanthate. The cellulose xanthate was dissolved in an aqueous alkali solution to yield a viscose containing 5% cellulose and 6% sodium hydroxide. The ball fall viscosity in seconds of this viscose was determined after ripening for 48 hours.

For comparison purposes, a second experiment starting with sulfur-free pulp was carried out in which 1% sulfur was added at the alkali cellulose shedding stage. A third experiment in which no sulfur was added was also carried out. Care was taken to insure that temperatures and times were identical at each stage of the three experiments. Results were as follows:

TABLE IIL-AGING INHIBITION CAUSED BY VARIOUS AMOUNTS OF SULFUR 0.10% by weight of sulfur is also an active inhibitor, as was illustrated following addition in CS solution to a prehydrolyzed hardwood sulfate pulp of dissolving grade. The CS was completely removed by drying before contact with caustic. In this preliminary experiment, only the viscose viscosity was determined. The viscose from treated pulp had a viscosity of 122 seconds, while an untreated control had a viscosity of 116 seconds.

Example V Aging was inhibited when a prehydrolyzed sulfate pulp (specifically Tyrecell grade pulp manufactured at the Natchez Mill of International Paper Company) and cotton linters pulp were used as cellulosic raw materials. All conditions in the following runs, including method of TABLE I.-EFFEOT OF 1% SULFUR ON THE AGING REACTION Reduction of chain shortening with sulfur is indicated, as is the advantage of early addition of the sulfur.

sulfur addition to pulp and aging temperature, were as in Example I.

TABLE V.INHIBITION USING LINTERS AND PREHYDROLYZED SUL FATE PULP D.P. after- Viscose Percent 24 viscosity Pulp sulfur Pressing Shredding hours (sec.) Prehydrolyzed sulfate 1 730 721 571 50. 6 D 0 728 722 549 31. 6 1 632 620 520 28. 6 0 639 625 469 15. 2

Example II Example VI The sulfur used in Example I was an unpurified grade such as is commonly used in the preparation of wood pulp. Two purer grades have also been tried at the 1% level while in CS solution in an experiment using the techniques given in Example I.

The purified sulfite pulp of Example I in sheet form was soaked in water overnight. Each soaked sheet was then slurried in water by 5 seconds stirring with a high speed mixer of the Osterizer type. Sulfur (1%) was added in dry form to the pulp slurry which was stirred a. further TABLE II.-RESULTS USING PURIFIED SULFUR Table II shows that the degree of inhibition effected by these purer grades of sulfur is of the same order as found in Example I.

Example III Using the pulp and procedures of Example I, the effect of the addition of different amounts of U.S.P. grade sulfur to pulp was studied.

15 seconds at the high speed. The slurry, which contained 0.3% cellulose, was diluted with ten volumes of water using hand or mild mechanical stirring for 30 seconds. The diluted pulp slurry was poured onto a screen, and excess Water allowed to drain. Drained water was used in the preparation of the next sheet. Make up water only was added. The wet sheet on the screen was pressed and 6 passed through a set of wringers of the type used in home dissolved in caustic to give viscoses containing 7% celwashing machines to remove all loosely retained water. lulose and 6% NaOH. The viscoses had ball fall viscosi- The new pulp sheets were dried, conditioned, and procties of 38 seconds. If both alkali cellulose samples had essed essentially in the same manner as in Example I. been aged at 25 0, experience shows that 106 and 67 The control sheets were prepared as above except that hours would have been required to reach a viscosity of no sulfur was added and fresh water was used. Water 5 38 seconds. Using the formula remaining after pre aration of both the control and sulfur handsheets was tesi ed for sulfur by shaking with sufur- Percent change In agmg:

free carbon disulfide. A drop of pure mercury added to (Aging time su1furaging time control) 100 the carbon disulfide did not turn black or change color in 10 Aging time control any way in either case. This proves that there was little hi corresponds to 58% longer aging when sulfur is or no contamination of the water with sulfur (Feigl: Spot used at the 1% level, based on cellulose Tests in Inorganic Analysis, 5th edition, 374, Elsev r Viscose color and transparency, cellulose yield, and

pubhshlflg f wsheet steeping properties (such as floating, cracking, and Caustlc dralned and expressed from the above Sheets processing absorption-all well known to those learned during alkali cellulose pressing was retained and reused i th rt of cellulose sheet steeping) were all identical in steeping a second batch of the original sheets of the between the samples. The two viscoses filtered with equal same pulp. ease and were spun after ripening into a bath containing TABLE IL-RESULTS OBTAINED USING SULFUR-CONTAINING HAND- SHEETS AND AFTER REUSING EXPRESSED CAUS'IIC (COMBINED DRAININ GS AND PRESSINGS) The results from the first three lines of the table clearly 130 g. of H 50 16 g. of ZnSO and 280 g. of Na SO show the inhibiting effect of the sulfur. The results obper liter. The rayons had excellent color and strength tained when expressed caustic was used with fresh, unafter washing and drying. No differences were found. dispersed pulp (last three lines of Table VI) show that little or no sulfur leaves the alkali cellulose during caus- Example ti al, Th generally hi h l l of re lt f Percent increase 1n viscose viscosity has been deterrecovered caustic when compared to the control handfilmed for y of the inhibtors mentioned in h rsheets results indicates that catalytic iron originally presu he ulfite pulp used in most of the above examem (5 ha b e v d f th di b 4 ples was used here also and percent increases were droxide solution by the .pulp in the previous steeping. 0 calculated by the formula:

E l VII Percent viscose viscosity increase= It is shown by Example IV that sulfur inhibits alkali 100 (Viscose Viscosity, inhibitOrviscose viscosity, control) cellulose aging but does not influence the catalytic effect Viscose viscosity, control of heavy metal impurities in caustic or pulp. This was illustrated further when a purer caustic is used in steep- All processing parameters were as given in Example I ing. This caustic contained 0.7 ppm. iron rather than and viscosities of control batches varied by not more the 5 ppm. present in the caustic used in all other than i4.0 seconds. exampes' 5O estes-estates re reates? F OTHER A- TABLE RESULTS WHEN PURE CAUSTIC WAS USED TERIALS MENTIONED AS INHIBITORS IN THE LITERA- DP aiter- TUBE Viscose Amount ofsulfur used (perh 24 vicosity gg cent of cellulose weight) Pressing Shreddlng ours (sec.) I Amount Viscosity 778 772 598 no Inhibitor used Where added increase 785 775 538 2 Sodium borohydride- 1% Reacted with pup 17. 3 Do .05%. do 0 Note by comparison with Table I, that the control 24 Copper ethylxantlate--z5o -D- Pyrogallol 1% 60. 0 hour DP. and viscose viscosity are higher in Table VII, Do. 4 1% 26.2 due to use of caustic containing less catalytic iron. Howsodlmgl Sulfite at? ever, the inhibitor effectiveness judging by the viscosity i ifl' fiifiie 7?, 1 and 24 hour D.P. increase achieved with the presence of DO 2%.; sulfur is almost unchanged. Had the sulfur acted by prevention of iron catalysis, very little inhibition should 3;. 5 have been found in the presence of only 0.7 ppm. of 1% 1'75 Iron 1 No inhibition was found when 1.0 and 0.05% sodium borohydride Example VIII was added to the caustic at steeping time. 2 Large amounts are not practical because of reagent insolubility. The sulfite dissolving grade wood pulp was treated with jzAs lolyllnflnqpliglflN OH 1 K li lf in causltic, Shredded As'iii vi n iii i ireh Uis. Pat. No; 1,689,958 to Moro.

c tr w s a so rocesse us1n e p e su ur ree on o a p g Example X same conditions. No samples were taken for D.P. determination. A sample of sulfite dissolving pulp, specifically Filmcell The sulfur-containing pulp was aged for three days at H, manufactured at the Hawkesbury mill of the Cana- 28 C. and the sulfur-free pulp was aged for three days dian International Paper Company, was treated with a at 23 C. The aged alkali celluloses were xanthated and solution of sulfur in carbon disulfide. The added weight of sulfur was 1% based on cellulose. This pulp was dried until no trace of carbon disulfide remained and was then converted to alkali cellulose by slurry steeping in 18% sodium hydroxide. After draining and pressing the alkali cellulose, containing 34% cellulose, was shredded and aged for three days at 25 C. After this time, the alkali cellulose was reacted with ethylene oxide gas (11% based on cellulose) simply by rotating the glass storage vessel at room temperature while connected to another vessel containing the required amount of liquid ethylene oxide. The ethylene oxide vaporized and was completely absorbed into, and reacted with, the alkali cellulose after two hours. The hydroxyethyl cellulose was dissolved in dilute caustic soda to give a solution containing 8% hydroxyethyl cellulose and 7% NaOH. The solution was frozen overnight, thawed, and allowed to come to 20 C. before the viscosity was determined.

A control batch of the same pulp was converted to alkali cellulose, then to hydroxyethyl cellulose, and dissolved simultaneously and using identical procedures to those used for the sulfur-containing pulp. The hydroxyethyl cellulose solution prepared using the pulp containing 1% sulfur had a viscosity of 52.6 secs. while the control batch containing no sulfur had a viscosity of 12.2 secs.

What is claimed is:

1 A stable composition comprising a cellulosic material, alkali, and sulfur in an amount of from about 0.1% to 2.0% by weight of the cellulosic material.

2. A composition as in claim 1 wherein the cellulosic material is selected from the group consisting of cotton linters, wood pulp and alkali cellulose.

3. A composition as in claim 2 wherein the sulfur is present in an amount of about 1% :by weight.

References Cited UNITED STATES PATENTS 5/19321 Dosne 260-233 6/1949 Kayser 260--233 US. Cl. X.R. 26023l, 233 

